Immunity is concerned with the recognition and disposal of foreign antigenic material which is present in the body. Typically the antigens are in the form of particulate matter (i.e., cells, bacteria, etc.) or large protein or polysaccharide molecules which are recognized by the immune system as being "non-self", i.e., detectably different or foreign from the animals own constituents. Potential antigens can be a variety of substances, often proteins, which are most frequently located on the outer surfaces of cells. For example, potential antigens can be found on pollen grains, tissue grafts, animal parasites, viruses, and bacteria. Once the antigenic material is recognized as "non-self" by the immune system, natural (non-specific) and/or adaptive immune responses can be initiated and maintained by the action of specific immune cells, antibodies and the complement system. Under certain conditions, including in certain disease states, an animal's immune system will recognize its own constituents are "non-self" and initiate an immune response against "self" material.
An immune response can be carried out by the immune system by means of natural or adaptive mechanisms, each of which are composed of both cell-mediated and humoral elements. Natural mechanisms for immune response refer to those mechanisms involved in essentially non-specific immune reactions which involve the complement system and myeloid cells alone, such as macrophages, mast cells and polymorphonuclear leukocytes (PMN), in reacting to certain bacteria, viruses, tissue damage and other antigens. These natural mechanisms provide what is referred to as natural immunity. Adaptive mechanisms for immune response refer to those mechanisms which are mediated by lymphocytes (T and B cells) and antibodies which can respond selectively to thousands of different materials recognized as "non-self". These adaptive mechanisms provide what is referred to as adaptive immunity and lead to a specific memory and a permanently altered pattern of response in adaptation to the animal's own environment. Adaptive immunity can be provided by the lymphocytes and antibodies alone or, more commonly, can be provided by the interaction of lymphocytes and antibodies with the complement system and myeloid cells of the natural mechanisms of immunity. The antibodies provide the humoral element of the adaptive immune response and the T-cells provide the cell-mediated element of the adaptive immune response.
Natural mechanisms of immune response involve phagocytosis by macrophages and PMN whereby foreign material or antigen is engulfed and disposed of by these cells. In addition, macrophages can kill some foreign cells through its cytotoxic effects. The complement system which is also involved in natural immunity is made up of various peptides and enzymes which can attach to foreign material or antigen and thereby promote phagocytosis by macrophages and PMN, or enable cell lysis or inflammatory effects to take place.
Adaptive mechanisms of immune response involve the actions against specific antigens of antibody secreted by B-lymphocytes (or B-cells) as well as the actions of various T-lymphocytes (or T-cells) on a specific antigen, on B-cells, on other T-cells and on macrophages.
Antibodies, which are responsible for the humoral aspect of adaptive immunity, are serum globulins secreted by B-cells with a wide range of specificity for different antigens. Antibodies are secreted in response to the recognition of specific antigens and provide a variety of protective responses. Antibodies can bind to and neutralize bacterial toxins and can bind to the surface of viruses, bacteria, or other cells recognized as "non-self" and thus promote phagocytosis by PMN and macrophages. In addition, antibodies can activate the complement system which further augments the immune response against the specific antigen.
Lymphocytes are small cells found in the blood which circulate from the blood, through the tissues, and back to the blood via the lymph system. There are two major subpopulations of lymphocytes called B-cells and T-cells. B-cells and T-cells are both derived from the same lymphoid stem cell with the B-cells differentiating in the bone marrow and the T-cells differentiating in the thymus. The lymphocytes possess certain restricted receptors which permit each cell to respond to a specific antigen. This provides the basis for the specificity of the adaptive immune response. In addition, lymphocytes have a relatively long lifespan and have the ability to proliferate clonally upon receiving the proper signal. This property provides the basis for the memory aspect of the adaptive immune response.
B-cells are the lymphocytes responsible for the humoral aspect of adaptive immunity. In response to recognition of a specific foreign antigen, a B-cell will secrete a specific antibody which binds to that specific antigen. The antibody neutralizes the antigen, in the case of toxins, or promotes phagocytosis, in the case of other antigens. Antibodies also are involved in the activation of the complement system which further escalates the immune response toward the invading antigen.
T-cells are the lymphocytes responsible for the cell-mediated aspect of adaptive immunity. There are three major types of T-cells, i.e., the Cytotoxic T-cells, Helper T-cells and the Suppressor T-cells. The Cytotoxic T-cells detects and destroys cells infected with a specific virus antigen. Helper T-cells have a variety of regulatory functions. Helper T-cells, upon identification of a specific antigen, can promote or enhance an antibody response to the antigen by the appropriate B-cell and it can promote or enhance phagocytosis of the antigen by macrophages. Suppressor T-cells have the effect of suppressing an immune response directed toward a particular antigen.
The cell-mediated immune response is controlled and monitored by the T-cells through a variety of regulatory messenger compounds secreted by the myeloid cells and the lymphocyte cells. Through the secretion of these regulatory messenger compounds, the T-cells can regulate the proliferation and activation of other immune cells such as B-cells, macrophages, PMN and other T-cells. For example, upon binding a foreign antigen, a macrophage or other antigen presenting cell can secrete interleukin-1 (IL-1) which activates the Helper T-cells. T-cells in turn secrete certain lymphokines, including interleukin-2 (IL-2) and .gamma.-interferon, each of which have a variety of regulatory effects in the cell-mediated immune response. Lymphokines are a large family of molecules produced by T-cells (and sometimes B-cells) including
IL-2, which promotes the clonal proliferation of T-cells;
MAF or macrophage activation factor, which increases many macrophage functions including phagocytosis, intracellular killing and secretion of various cytotoxic factors;
NAF or neutrophil activation factor, which increases many functions of the PMN including phagocytosis, oxygen radical production, bacterial killing, enhanced chemotaxis and enhanced cytokine production;
MIF or macrophage migration factor, which by restricting the movement of macrophages, concentrates them in the vicinity of the T-cell;
.gamma.-interferon, which is produced by the activated T-cell and is capable of producing a wide range of effects on many cells including inhibition of virus replication, induction of expression of class II histocompatibility, molecules allowing these cells to become active in antigen binding and presentation, activation of macrophages, inhibition of cell growth, induction of differentiation of a number of myeloid cell lines.
Activated macrophages and PMNs, which provide an enhanced immune response as part of the cell-mediated adaptive immunity, are characterized as having increased production of reactive oxygen intermediates. This increased production of reactive oxygen intermediates, or respiratory burst, is known as "priming". Certain lymphokines, such as .gamma.-interferon, trigger this respiratory burst of reactive oxygen intermediates in macrophages and PMNs. Thus, lymphokines, such as .gamma.-interferon, which are secreted by the T-cells provide an activation of these macrophages and PMNs which results in an enhanced cell-mediated immune response.
The immune response can provide an immediate or a delayed type of response. Delayed-type hypersensitivity is an inflammatory reaction which occurs in immune reactive patients within 24-48 hours after challenge with antigen and is the result primarily of a cell-mediated immune response. In contrast, immediate-type hypersensitivity, such as that seen in anaphylactic or Arthus reactions, is an inflammatory reaction which occurs in immune reactive patients within minutes to a few hours after challenge with antigen and is the result primarily of humoral or antibody-mediated immune response.
The ability of the immune system, and in particular the cell-mediated immune system, to discriminate between "self" and "non-self" antigens is vital to the functioning of the immune system as a specific defense against invading microorganisms. "Non-self" antigens are those antigens on substances in the body which are detectably different or foreign from the animals own constituents and "self" antigens are those antigens which are not detectably different or foreign from the animals own constituents. Although the immune response is a major defense against foreign substances which can cause disease, it cannot distinguish between helpful and harmful foreign substances and destroys both.
There are certain situations, such as with an allogeneic transplant or in "graft versus host" disease, where it would be extremely useful to suppress the immune response in order to prevent the rejection of helpful foreign tissue or organs. Allogeneic tissues and organs are tissues and organs from a genetically different member of the same species. "Graft versus host" disease occurs where the transplanted tissue, for example in a bone marrow transplant, contains allogeneic T-cells of the donor which cause an immune response against the recipient's own tissues. Although both humoral and cell-mediated immune responses play a role in the rejection of allogeneic tissues and organs, the primary mechanism involved is the cell-mediated immune response. Suppression of the immune response, and in particular, suppression of cell-mediated immune response, would thus be useful in preventing such rejection of allograft tissues and organs. For example, cyclosporin A is currently used as an immunosuppressive agent in the treatment of patients receiving allogeneic transplants and in "graft versus host" disease.
There are times when the individual's immunological response causes more damage or discomfort than the invading microbes or foreign material, as in the case of allergic reactions. Suppression of the immune response in these cases would be desirable.
Occasionally, the immunological mechanisms become sensitized to some part of the individual's own body causing interference with or even destruction of that part. The ability to distinguish between "self" and "not self" is impaired and the body begins to destroy itself. This can result in an autoimmune diseases such as rheumatoid arthritis, insulin-dependent diabetes mellitus (which involves the autoimmune destruction of the .beta.-cells of the islets of the Langerhans which are responsible for the secretion of insulin), certain hemolytic anemias, rheumatic fever, thyroiditis, ulceractive colitis, myestheniagravis, glomerulonephritis, allergic encephalo-myelitis, continuing nerve and liver destruction which sometimes follows viral hepatitis, multiple sclerosis and systemic lupus erythematosus. Some forms of autoimmunity come about as the result of trauma to an area usually not exposed to lymphocytes such as neural tissue or the lens of the eye. When the tissues in these areas become exposed to lymphocytes, their surface proteins can act as antigens and trigger the production of antibodies and cellular immune responses which then begin to destroy those tissues. Other autoimmune diseases develop after exposure of the individual to antigens which are antigenically similar to, that is cross-react with, the individual's own tissue. Rheumatic fever is an example of this type of disease in which the antigen of the streptococcal bacterium which causes rheumatic fever is cross-reactive with parts of the human heart. The antibodies cannot differentiate between the bacterial antigens and the heart muscle antigens and cells with either of those antigens can be destroyed. Suppression of the immune system in these autoimmune diseases would be useful in minimizing or eliminating the effects of the disease. Certain of these autoimmune diseases, for example, insulin-dependent diabetes mellitus, multiple sclerosis and rheumatoid arthritis, are characterized as being the result of a cell-mediated autoimmune response and appear to be due to the action of T-cells [See Sinha et al. Science 248, 1380 (1990)]. Others, such as myestheniagravis and systemic lupus erythematosus, are characterized as being the result of a humoral autoimmune response [Id.].
Suppression of the immune response would thus be useful in the treatment of patients suffering from autoimmune diseases. More particularly, suppression of cell-mediated immune response would thus be useful in the treatment of patients suffering from autoimmune diseases due to the action of T-cells such as insulin-dependent diabetes mellitus, multiple sclerosis and rheumatoid arthritis. Suppression of humoral immune response would be useful in the treatment of patients suffering from T-cell independent autoimmune diseases such as myestheniagravis and systemic lupus erythematosus.